How can I word my request for overtime pay to my boss?

He asked if I can come on weekend to put in extra hours finishing something. I already work non-stop during the workweek. But he never mentioned pay, and I kept quiet and didn't even ask. Yesterday I went and worked for a few hours (on Valentine's day that too) and now I am feeling a bit resentful and I don't want to be taken advantage of. I am already getting a v. mimimal salary as it is and instead of hiring a part-time help, or paying more, he is over-using me I feel. I take care of the entire dept by myself. I do have my own life on weekends. I wouldnt mind putting in more hours for pay. How can I Word the request for over-time pay without sounding greedy for money. And is it Right for him to ask me to work weekends with no pay? I am on salary (not hourly).